linux-kernel - Re: [PATCH] ftrace: use only 5 byte nops for x86

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Date:	Fri, 15 Aug 2008 11:22:31 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Miller <davem@...emloft.net>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Roland McGrath <roland@...hat.com>,
	Ulrich Drepper <drepper@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Gregory Haskins <ghaskins@...ell.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	"Luis Claudio R. Goncalves" <lclaudio@...g.org>,
	Clark Williams <williams@...hat.com>, srostedt@...hat.com
Subject: Re: [PATCH] ftrace: use only 5 byte nops for x86


* Jeremy Fitzhardinge <jeremy@...p.org> wrote:

> Steven Rostedt wrote:
>> Mathieu Desnoyers revealed a bug in the original code. The nop that is
>> used to relpace the mcount caller can be a two part nop. This runs the
>> risk where a process can be preempted after executing the first nop, but
>> before the second part of the nop.
>>
>> The ftrace code calls kstop_machine to keep multiple CPUs from executing
>> code that is being modified, but it does not protect against a task preempting
>> in the middle of a two part nop.
>>
>> If the above preemption happens and the tracer is enabled, after the
>> kstop_machine runs, all those nops will be calls to the trace function.
>> If the preempted process that was preempted between the two nops is executed
>> again, it will execute half of the call to the trace function, and this
>> might crash the system.
>>
>> This patch instead uses what both the latest Intel and AMD spec suggests.
>> That is the P6_NOP5 sequence of "0x0f 0x1f 0x44 0x00 0x00".
>> Note, some older CPUs and QEMU might fault on this nop, so this nop
>> is executed with fault handling first. If it detects a fault, it will then
>> use the code "0x66 0x66 0x66 0x66 0x90". If that faults,
>
> Seems very unlikely, but go mad ;)
>
>>  it will then
>> default to a simple "jmp 1f; .byte 0x00 0x00 0x00; 1:". The jmp is
>> not optimal but will do if the first two can not be executed.
>>
>> TODO: Examine the cpuid to determine the nop to use.
>>   
>
> Don't think that will help in general.  qemu claims its a Pentium II.

ok. I've applied Steve's patch as it's a good bugfix. The extra paranoia 
about faulting on 0x90 seems excessive but doesnt hurt ;-)

	Ingo
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